The present invention relates to a recording apparatus which records data on a recording medium, and particularly to a recording apparatus in which suction of the recording medium is devised.
Generally, in a large-sized printer that is one of conventional recording apparatuses, a sheet feeding portion for feeding a recording medium, for example, printing roll sheet, a printing unit for printing data on the fed roll sheet, and a sheet discharging portion for discharging the printed roll sheet are arranged in this order from the upper part. In case that this large-sized recording apparatus, for example, an ink jet printer is used, a user houses a roll sheet in the sheet feeding portion and pulls out a leading end of the roll sheet. And, the user lets the leading end of the roll sheet pass through a flat sheet feeding guide that functions as a sheet transporting surface, and then interposes it between a sheet feeding roller and a driven roller to start the printer.
Then, the ink jet printer, while it rotates the sheet feeding roller and feeds out the roll sheet on a flat platen that functions as a sheet transportation guide surface, ejects ink droplets from a nozzle opening of a printing head and prints data on the roll sheet. Thereafter, the printer rotates the discharging roller and discharges the roll sheet to the outside through a flat sheet discharging guide that functions as a sheet transporting surface.
In such the conventional printers, in order to prevent the curled roll sheet from rising after printing, there is provided a sheet suction portion which sucks the roll sheet between the platen and the sheet discharging guide. In this sheet suction portion, plural suction ports are provided in the direction perpendicular to the transporting direction of the roll sheet, i.e., in the main scanning direction, and in the transporting direction of the roll sheet, i.e., in the sub-scanning direction; and fans for sucking the outside air from each suction ports are included.
However, in case that many suction ports are provided as described above, the suction capacity of the fans decrease in case the number of the arranged fans is limited and particularly the leading end of the greatly curled roll sheet cannot be sucked reliably. While reduction of the number of the suction ports resolves this problem, some suction ports are required since the roll sheet is large, so that it is difficult to balance the number of the suction ports with the suction capacity of the fans.
Generally, a sheet has the property of curving easily due to absorption. Particularly, in case that the large-sized roll sheet, for example, a wide roll sheet of A0 size or B0 size in JIS (Japanese Industrial Standard) is curved, rising of the roll sheet is easy to be produced on the platen in the printing unit, so that there is fear that printing accuracy cannot be kept. Therefore, the suction unit is provided in order to suck the roll sheet on the sheet transporting surface, whereby the rising of the roll sheet on the platen is prevented and the printing accuracy is secured.
However, when the roll sheet is curled greatly, it cannot be sucked on the sheet transporting surface by the suction unit. Particularly, when such the roll sheet is set, the leading end of the roll sheet does not frequently pass through the discharging roller. Further, when the roll sheet has little rigidity and is thin, it is stuck onto the sheet transporting surface by the suction unit. Particularly, when such the roll sheet is set, it cannot be moved to its set position.
Further, not only in the large-sized printers which carries out recording on a roll sheet, but also in normal inkjet printers, when a large number of ink droplets are ejected on a sheet, for example, a solid image is recorded on the sheet, the sheet absorbs a large amount of ink and is swollen like a wave onto a recording head side after the recording, that is, cockling is generated. When the cockling is generated and grown, an interval between the sheet and the recording head becomes nonuniform so that the flight distance of the ink droplet is varied to cause a recording unevenness or the sheet comes in contact with the recording head and is thus contaminated. In recent years, there has been proposed an ink jet printer for forming, on a sheet transportation surface, a plurality of holes at a constant pitch in a sheet transporting direction and a direction perpendicular thereto, that is, a plurality of grid-like holes and sucking a sheet by means of a sucking pump through these holes, thereby suppressing the cockling (see JP-A-63-303781 and JP-A-3-270).
In the conventional ink jet printer comprising a sucking type sheet transportation apparatus, a sucking drag to be a transportation resistance of a recording medium which is generated by a suction is changed due to a variation in the area of a sheet covering the hole or a difference in a coefficient of friction for each type of the sheet so that the transportation amount of the sheet is varied, resulting in a deterioration in recording precision in some cases.